The prior art evidences a variety of arrangements for the purpose of controlling the motion of an object, either its position as a function of an independent parameter, its velocity as a function of an independent parameter, or both. A simple arrangement to effect position control is illustrated in Davis, U.S. Pat. No. 3,099,777. In Davis, a bi-directional counter is initially loaded with a count identifying the controlled object's present position, and a second counter is loaded with a count corresponding to the desired position of the controlled object. The second counter is down counted to zero, while simultaneously down counting the first counter. When the second counter has been counted to zero, the quantity remaining in the first counter is the offset or difference between the present and desired positions of the object. At that time, a gate coupled between a digital to analog converter, responsive to the output of the first counter and a motor is enabled to couple the output of the digital to analog converter to the motor to thereby drive the motor toward the desired position. A feedback circuit is used to modify the count in the first counter in accordance with actual movement of the object, such that the object is brought to rest at the desired position. Thus, the Davis arrangement provides for movement of the object from a present to a desired position, although it makes no provision for the control of that movement with respect to an independent parameter; in other words, the acceleration or velocity imparted to the controlled object is controlled solely by the extent through which the object is to be moved, and there is no control over the time duration of the movement itself and no control over the shaping of the acceleration, velocity and position profiles.
Gardener, in U.S. Pat. No. 3,859,581, discloses a technique for controlling jerk or rate of change of acceleration in which the rate at which a motor control signal can change is controlled. The motor control signal is provided by the digital to analog converted output of a bi-directional counter, which is counted up or down at a rate determined by a clock in accordance with the relationship between actual and desired velocities of a controlled object. Gardener teaches that the clock frequency can be controlled so as to put a limit on maximum acceleration.
Reuteler, in U.S. Pat. Nos. 3,414,787 and 3,443,178 discloses a servo system for controlling position and velocity of a controlled object. The servo system includes for each axis of movement of the controlled object, both a velocity and a position loop so that the gain of the position loop can be adjusted without affecting the velocity loop. The input commands to the servo system include a displacement quantity, one for each independent axis of movement, and a common feed rate quantity intended to determine the rate of movement with respect to all axes. The feed rate number is added to itself a number of times at a selected rate, and when the sum exceeds a predetermined quantity, an overflow pulse is produced. An overflow pulse train is provided to a command pulse generator which is a counter which counts overflow pulses, and various stages of the counter are used to generate a variety of pulse frequencies. The largest pulse frequency is employed to generate command pulses, proportional in number to the desired displacement, and produced at a rate related to the feed rate number. Thus, in contrast to Davis and Gardener, the Reuteler system controls both velocity and position, although employing a different loop for each parameter.
Foster, in U.S. Pat. No. 4,066,941 discloses an arrangement useful in speed control. In several embodiments of the invention, the controlled motor is a stepping motor and speed control is effected by varying the inter-pulse periods of the several stepping pulses used to drive the motor. The inter-pulse periods are varied by deriving those pulses from a counter which is clocked at a constant rate, but which may be preset after reaching a predetermined count, to a value which is either the complement of, or directly related to the desired inter-pulse period. In another embodiment of the invention in which a continuously rotating motor is controlled, a read-only memory stores the quantities which are either directly related to, or complements of the desired inter-pulse period and which are successively coupled to a comparator at the completion of each incremental movement of the motor shaft. A constant frequency oscillator clocks a counter which is also reset on the completion of a given increment of movement of the motor shaft. The comparator compares the quantities presented to it by the read-only memory and the timing counter, and can, from those quantities, determine whether the motor is travelling at the proper speed, above the proper speed, or below the proper speed. The output of the comparator is employed to control the driving current of the motor in a fashion so as to reduce the "error" in motor speed. While Foster appears to provide an effective arrangement to control speed, this apparatus makes no provision for controlling the position profile of the motor.
Maeda, in U.S. Pat. No. 4,145,643, discloses an arrangement for driving a pulse motor in which a command, having a quantity related to the desired movement of the motor, is decremented as the motor is rotated, and in which the frequency at which the quantity is decremented, and hence, the velocity of the motor is determined by the ratio of the distance the motor is to be moved to the time over which the movement is to occur. Hence, an arrangement is disclosed for controlling the velocity of a motor.
As indicated by the Reuteler patent, merely controlling the velocity of movement is, in many applications, inadequate. One such application in which merely controlling the velocity of an object is inadequate is that of driving the scanning carriage in a continuously variable reducing copying machine, such as that disclosed in application Ser. No. 100,775 filed concurrently with this application, and assigned to the assignee of this application.
As is disclosed in that application, correct operation of the continuously variable reducing copying machine requires that a scanning carriage be driven in a cyclical movement from a home position, first to a rescan position, the length of the rescan depending, at least in part, upon the selected reduction ratio, and that the scanning carriage then be scanned back toward the home position in such a manner that it reach a velocity having a ratio with the velocity of the copier drum equal to the selected reduction ratio, and maintain that relationship throughout a given scanning movement, and then be decelerated to stop at the home position. However, merely controlling the velocity of the carriage is inadequate since not only must the velocity of the carriage reach the desired velocity relationship, but that relationship must be reached at a given position in the movement, and at the correct time. Thus, devices for merely controlling the velocity of an object simply fail to satisfy the requirements of driving the scanning carriage in a continuously variable reducing electrophotographic copier.
The referred-to application discloses two preferred embodiments, a first preferred embodiment which employs three (3) partially overlapping feedback loops in order to control the motor driving the scanning carriage assembly. A first feedback loop derives a pulse train at a repetition rate related to the velocity of the copier drum, which is used to cycle a counter, which counter is preset so that it produces an output pulse when the counter reaches a predetermined quantity, which output pulse forms a pulse in a pulse train having a repetition rate bearing a selected ratio with the pulses cycling the counter, which ratio is selected in accordance with the desired velocity ratio between the scanning carriage assembly and the copier drum. The pulse train from the counter is applied to a phase comparator which receives on another input pulses provided by a tachometer driven by the motor driving the scanning carriage assembly. The output of the phase comparator thus provides a representation of the velocity error, which is used in a velocity feedback loop to maintain a velocity error near zero. In order to insure that the scanning carriage assembly reaches the proper velocity at the proper point in time, relative to the copier drum, a second feedback loop is employed to control acceleration and deceleration of the scanning carriage assembly. In this second feedback loop, a processor stores a table of desired scanning carriage assembly positions, as a function of time during the acceleration or deceleration phases of movement. At periodic intervals, determined in relation to the rotational rate of the copier drum, a counter which counts pulses from the tachometer is compared to a count representing the desired position of the scanning carriage assembly. Any difference or error is employed to modify an accelerating drive signal, which is used to drive the motor during its acceleration phase. Thus, the second feedback loop insures that the scanning carriage assembly reaches the desired velocity at the proper point in time, i.e., its position profile is controlled. The count attained in the counter employed in the acceleration feedback loop is maintained during the constant velocity phase, and another counter, keeping track of scanning carriage assembly position, continuously compares the two quantities. When the counts compare, the constant velocity phase of motion is terminated, and a deceleration phase is entered. When deceleration is complete, in a rescan for example, the several counters are reset and the scanning movement is initiated, having similar acceleration, constant velocity and deceleration phases of movement.
While the first preferred embodiment briefly described above provides for sufficient control of the scanning carriage assembly for use in a continuously variable reducing copying machine, the present application discloses an improved driving arrangement for such a scanning carriage assembly which exhibits significant advantages over the first preferred embodiment of the referenced application.
The control philosophy of the first preferred embodiment of the referenced application is based upon the desired goal of employing equal acceleration of the scanning carriage assembly regardless of the reduction ratio. The use of a fixed acceleration, although resulting in some simplifications in the control circuit, can be disadvantageous in some respects, especially in the initial stages of the movement, for it results in a relatively large and rapid change of acceleration, requiring sufficient power handling capacity of a motor. We have found that by tailoring the acceleration profile of the motor, components having a reduced power handling capacity are adequate. In addition, by tailoring the acceleration profile of the motor, physical resonances in the mechanical components can be reduced, thus significantly increasing the accuracy with which the movement is carried out.
In addition, typical commercial copiers employ a copy sequence in which the drum rotates twice for each copy produced; actual copying takes place on a first drum cycle, and the second drum cycle is devoted to cleaning the drum. By use of an overcharge/backcharge technique described in Ser. No. 93,551 filed Nov. 3, 1979, two cycle machines can be changed into a one cycle machine, i.e., a machine capable of producing a copy on each rotation of the drum as long as the document to be copied is not changed. There is a need to reimage the document on each cycle and the image on the drum must coincide precisely with the preceding image to avoid smearing the image. By increasing the accuracy with which the movement of the scanning carriage assembly is effected, we have found that we can control the registration of the image on the copying drum in different cycles within a tolerance conservatively set at .+-.0.1 mm. Obviously, a copier which does not waste drum revolutions in cleaning the drum is desirable. However, the first preferred embodiment of the referenced application did not have a position tolerance small enough to enable the production of quality copy in this environment. In the brief description of the first preferred embodiment, mention was made of a counter which was initially loaded with a count representing the desired length of movement of the scanning carriage assembly for the selected reduction ratio, which counter was counted down during the course of either a rescan or a scan movement. In order to effect this operation, the counter had to be reset at the conclusion of a rescan operation and prior to the beginning of a scan movement. This resetting of the counter meant that any accumulated error in the position of the carriage at the termination of the rescan movement was lost, and that error would be reflected in an error in the scanning carriage assembly position profile. While this "error" could be eliminated by increasing the gain of the loop, this is not a practical alternative since it would increase the susceptibility of the system to physical resonance. Also, the processor employed to make the repeated comparisons of scanning carriage assembly position in the course of the accelerating movement is a poorly timed device which also introduced variations in scanning carriage position from one rescan-scan cycle to the next which limited achievable position repeatability. Similar errors may well have been introduced during the switching from one feedback loop to another during the transition from acceleration to constant velocity motion, and constant velocity motion to deceleration.
It is therefore one object of the present invention to provide a position control system which has the capability of simultaneously controlling position profile, acceleration profile and velocity profile with a single control loop. It is another object of the present invention to provide a driving control system which eliminates the necessity for relying upon a processor in the control loop. It is another object of the present invention to provide a driving control arrangement for simultaneously controlling position and velocity profiles of a controlled object in which the control object exhibits cyclical motion, and in which the driving control arrangement eliminates the necessity for resetting a counter representing controlled object position. It is another object of the present invention to provide such a driving control arrangement in which the acceleration, velocity and position profiles of the controlled object can be controlled with respect to an independent parameter, i.e., for example, the velocity of a copying drum. A further object of the present invention is to provide such a driving control arrangement in which the acceleration, velocity and position profiles of the controlled object is repeatable from cycle to cycle with a relatively small position tolerance, i.e., on the other of .+-.0.1 mm. or less. It is still another object of the present invention to provide a driving control arrangement which does not rely upon the use of constant acceleration or constant deceleration movement but which is capable of tailoring acceleration and deceleration to decrease the power handling requirements of various components of the system.
It is another object of the present invention to eliminate (or reduce the accuracy requirement of) critical adjustments. Another object of the present invention is to provide a simple closed loop and accurate means of determining the home position (following power on or as part of an error recovery procedure). Note that the first preferred embodiment of the referenced application uses an open loop means of determining the home position.
Another important object of the invention is to provide in a scanning copier of commercially acceptable print quality the ability to copy on each cycle of a drum by immediately at the conclusion of an image transfer, deceleration the carriage, reversing its motion and accelerating it again to bring it up to scanning velocity during the time in which the drum rotates from end of image area to the beginning of the image area. Another object of the invention is to perform the foregoing fast flyback operation and reimage the drum image area with position error less than 0.2 mm.